A fuel assembly in a nuclear reactor consists of an elongated tubular container, often with a rectangular or square cross section, which is open at both ends forming a continuous flow passage, through which the coolant of the reactor is able to flow. The fuel assembly comprises a larger number of also elongated tubular fuel rods, arranged in parallel in a certain definite, normally symmetrical pattern. At the top, the fuel rods are retained by a top tie plate and at the bottom by a bottom tie plate. To allow coolant in the desired manner to flow past the fuel rods, it is important that these be kept at a distance from each other and prevented from bending or vibrating when the reactor is in operation. For this purpose, a plurality of spacers are used, distributed along the fuel assembly in the longitudinal direction.
Known spacers often comprise an external spacer frame and, inside this spacer frame, plate bands arranged crosswise and standing on end, these plate bands forming substantially square cells. It is also common for the cells to be formed from sleeves. Inside the cells there are arranged fixed and resilient supports, respectively, for the fuel rods extending through the cells.
Since the coolant in a nuclear reactor of boiling-water type (BWR) boils, a ratio of water to steam is formed which varies axially in the core. The coolant flows from the bottom and upwards in the core. At the bottom of the core, the temperature of the cooling water is lower than the boiling temperature and is thus in single phase, that is, the space between the fuel rods in the lower part of the assembly is, in operation, filled with non-boiling water. Further up, where the coolant has reached the boiling temperature, water is transformed into steam and the coolant is in two phases. The further up in the core, the higher the proportion of steam in relation to the proportion of water. In the upper part of the core, the fuel rods are only covered with a thin film of water, outside of which steam mixed with water droplets flows.
If the heat flux from a fuel rod becomes very large in relation to the coolant flow, there may be a risk of dryout occurring, that is, the liquid film becomes so thin that it is not able to hold together. The liquid film is broken up and dry wall portions are formed, which locally leads to a considerably deteriorated thermal transmittance between the fuel rod and the coolant, resulting in a greatly increased wall temperature of the fuel rod. The increased wall temperature may lead to damage with serious consequences arising on the fuel rods. The risk of dryout is greatest in the upper part of the fuel where the percentage of steam is greatest.
Also the wall of the fuel assembly, that is, the inside of that fuel channel which surrounds four identical bundles of fuel rods retained by spacers, is coated by a water film. However, this film is not entirely necessary since the wall of the fuel channel is considerably more insensitive to superheating than the fuel rods.
The spacers influence the flow of the coolant and hence the cooling of the fuel. It is known that, in a region just below the spacer where the coolant has not yet passed the spacer, a deterioration of the water film on the fuel rods takes place, whereas in region above the spacer, where the coolant has just passed the spacer, a reinforcement of the water film instead occurs. The reinforcement of the water film is due to the turbulence which arises in the coolant when it passes a spacer. The greatest risk of dryout exists in the upper part of the fuel just below the spacers.
A limiting factor with respect to the dryout power, that is, the total power that can be obtained from the fuel assembly without the risk of dryout existing, is usually the power that is obtainable from the fuel rods arranged in the corners of the spacer. These fuel rods, in the following referred to as corner rods, are sensitive since they are surrounded by only a small quantity of coolant, which limits the load possibility while at the same time non-boiling water with a good moderating capacity is present outside the corners of the fuel channel, whereby the power in the corner rods tends to become too high. A lower power output from the corner rod can be achieved with a lower enrichment thereof. However, it is not desirable to lower the enrichment level too much since, at the same time, a uniform enrichment design in the fuel assembly and as high a power output as possible are aimed at.
One way of improving the cooling is to reduce the distance between the spacers, such that the distance becomes small in the upper half of the fuel bundle where the sensitivity is greatest. However, this requires that the pressure drop in the coolant does not increase too much across the spacers such that the cooling capacity decreases. Normally, the positive cooling capacity of a spacer does not reach all the way up to the next spacer. Dryout occurs, as described above, exactly below the nearest downstream spacer. When an extra spacer level is introduced in the bundle, the total pressure drop should not increase in the fuel bundle, which means that the pressure drop of the individual spacer in such a case must be reduced.
One object of the invention is to improve the cooling of the fuel rods, in particular of the fuel rods which are arranged at the corners of the spacers, and to improve the cooling of at least the upper part of the fuel rods so as to reduce the risk of dryout. A reduced risk of dryout means that the fuel can be utilized more efficiently, which entails economical advantages.